The present invention relates to a phase locked loop and, more particularly, to a phase locked loop which subtracts an estimate of the incoming signal from the actual signal to produce a residue signal that is then demodulated and utilized to control the loop.
The use of a phase locked loop in communication circuits in which a local oscillator is synchronized in phase and frequency with a received signal is well known. The technique of making the phase of the oscillator signal follow exactly the phase of an input reference signal by comparing the phases between the two signals and using the result of the difference signal to adjust the frequency of the local oscillator is also known. The adjustment is accomplished by comparing the input reference signal with the output of the local oscillator. Any phase difference between the two signals is converted to a correction voltage that causes the phase of the local oscillator output signal to change so that it tracks the input reference signal.
The output of the local oscillator multiplied by the input reference signal has a tendency to produce a ripple-like error signal. This ripple-like error signal is normally fed back into the local oscillator input control to produce a phase jitter at twice the frequency which is being generated by the phase locked loop.